U.S. Patent Application 61/188,434 filed on Jul. 31, 2008 and incorporated herein by reference describes a hearing aid system comprised of an implanted housing having a distal portion configured to extend percutaneously to a user's ear canal to locate both a sound producing transducer (e.g., speaker) and a sound responsive transducer (e.g., microphone) in, or immediately adjacent to, the user's ear canal. In order to minimize the effects of acoustic feedback, feedback cancellation electronics is incorporated between the sound producing transducer and the sound responsive transducer.
Acoustic feedback often occurs in hearing aid devices when sound picked up by the hearing aid microphone is amplified by the hearing aid speaker, fed back into the microphone and re-amplified. This results in very annoying oscillations, or whistling, which render the hearing aid useless. Such, feedback induced oscillation is particularly difficult to avoid in open canal hearing aids having high amplification gain.
Different approaches have been proposed for reducing such feedback induced problems, including simply reducing the hearing aid gain. This however restricts the application of the hearing aid to mild hearing impairments. More sophisticated approaches can use adaptive feedback cancellation to reduce the affects of acoustic feedback. For example, U.S. Pat. No. 6,876,751 uses an adaptive digital filter to estimate the feedback signal and subtract it from the hearing aid microphone input.
A known problem with such feedback cancellation techniques is that successful operation relies on uncorrelated input signals, ideally white noise. If there is correlation between the hearing aid input and output signals, bias will likely be introduced into the adaptive filter which can compromise performance and introduce artifacts. The high correlation of tonal inputs often leads to an erroneous estimation of the feedback signal and results in the tonal inputs being subtracted. In order to minimize such problems, a time delay can be introduced into the processing loop to reduce the correlation and prevent voice signals from being degraded. However, only very short delays (milliseconds) can be tolerated before it becomes noticeable.
Another approach is to cause the filter to adapt sufficiently slowly so that important tonal inputs are not degraded by feedback cancellation processing. The disadvantage of this approach is that the adaptive filter may not adapt quickly enough to follow sudden changes which can occur in the feedback path resulting in feedback oscillations that may last until the feedback has stabilized. Accordingly, it appears that fast adaptation speeds are desirable when the filter needs to adapt to sudden changes in the feedback path but slow adaptation speeds are desirable to preserve voice and tonal signal sound quality.
Sudden changes in the acoustic feedback path are likely to occur as a consequence of normal activities such as placing a cellular phone close to the user's ear or placing a hat on the user's head while the hearing aid is operating. Such sudden changes in the acoustic feedback path are likely to produce feedback induced oscillations unless the adaptive cancellation filter adapts fast enough to follow such changes. In order to avoid these oscillations, fast adaptation speeds are required for such dynamic situations, i.e., sudden changes in the acoustic feedback path.